Advances in polymerization and catalysis have resulted in the ability to produce many new polymers having improved physical and chemical properties useful in a wide variety of superior products and applications. With the development of new catalysts, the choice of polymerization (solution, slurry, high pressure or gas phase) for producing a particular polymer have been greatly expanded. Also, advances in polymerization technology have provided more efficient, highly productive and economically enhanced processes.
As with any new technology field, particularly in the polyolefins industry, a small savings in cost often determines whether a commercial endeavor is even feasible. The industry has been extremely focused on developing new and improved catalyst systems. Some have focused on designing the catalyst systems to produce new polymers, others on improved operability, and many more on improving catalyst productivity. The productivity of a catalyst, that is, the amount of polymer produced per gram of the catalyst, usually is the key economic factor that can make or break a new commercial development in the polyolefin industry.
Ziegler-Natta catalyst systems are utilized extensively in commercial processes that produce high density and low-density polyethylenes in a variety of molecular weights. Production rates in certain gas phase reactors may be limited in their ability to discharge from the reactor the polymer particles that are produced during the reaction. In certain of such cases, an increase in the bulk density of the polymer particles may increase the production rate of the reactor. Generally, Ziegler-Natta catalysts that have increasing activity and productivity, and that are used in gas phase operations. may tend to produce polymer products that have decreasing bulk density. If a reactor is limited in its ability to discharge the polymer product, the use of a high activity catalyst may result in a decrease in the bulk density of the polymer product.
Bimodal polymers produced using two or more different catalyst types—bimetallic catalysts—are of increasing interest, especially in producing polyethylene and other polyolefins. Improving catalyst productivity also is of concern here, as productivity should be as high as possible in order to optimize the economic efficiency of the process, given the significant cost of bimetallic catalysts.
Background references include U.S. Pat. Nos. 5,693,583; 5,550,094; 5,032,562; 5,882,750; 5,539,076; 6,194,520; and EP 0746574.